As more and more wastewater is discharged into water bodies around the world, nitrogen (N) and phosphorus (P) levels in water bodies increase dramatically, exceeding the self-purification level of the environment itself, resulting in algal microbial blooms; lower DO; higher turbidity and pH; the disappearance of aquatic plants, especially submerged plants; and mass mortality of fish, ultimately leading to the eutrophication of freshwater resources and the collapse of aquatic ecosystems ^[1][2][1,2]. According to the total phosphorus (TN), total nitrogen (TP), and chlorophyll a (Chl-a) concentrations and transparency, the degree of eutrophication of water bodies can be classified as, poor, light, medium, medium-rich, heavy, and exceptionally eutrophic [3]; see Table 1. Studies have shown that total nitrogen and total phosphorus are vital factors in inducing cyanobacterial water outbreaks [4]. When TN and TP exceed 0.5 mg·L⁻¹ and 0.02 mg·L⁻¹, respectively, they can promote water bloom formation.

With the increasing eutrophication of lakes, especially the increase in phosphorus concentration, cyanobacteria significantly dominate the phytoplankton community succession. Under the environment of global warming and the yearly increase in CO₂ and other greenhouse gas emissions, the scale and frequency of cyanobacterial blooms are expanding, which has attracted extensive attention and research from scholars at home and abroad. The outbreak of cyanobacterial blooms has caused many severe threats to human life and health, production, and living [5]. Visually, there is a reduction in water clarity and a deepening of color and turbidity [6], and olfactively, the water body emits a strong and irritating odor. In addition, the proliferation of cyanobacteria has led to the continuous deterioration of the water quality environment and eventually causes the collapse of freshwater ecosystems [7]. In waters with cyanobacterial bloom outbreaks, the dissolved oxygen (DO) levels in the water column are extremely low and the concentrations of cyanobacterial toxins (MCs) extremely high, and the biodiversity is severely damaged.

The current methods of removing cyanobacteria are mainly divided into chemical, physical, and biological methods, but they are relatively independent of each other ^[8][9][8,9]. In the face of large-scale cyanobacterial blooms, a single technology can provide a certain degree of removal, but there are some obvious drawbacks, and the effectiveness of different methods varies widely. Therefore, how to choose an effective algae removal technology to achieve the highest algae removal rate has become a key and difficult problem to be solved.

2. Physical Algae Removal Technology

The most commonly used techniques for algae removal by physical methods include mechanical methods, shading technology methods, air flotation, clay flocculation, ultrasonic methods, filtration, ultraviolet irradiation, and adsorption. The principle of the mechanical method is to use a power device to gather and salvage the algae in the lake, to quickly obtain a large amount of algal biomass and rapidly reduce the concentration of algae in the water body, which is one of the commonly used treatment means, but due to the high water content of the salvaged algae, it increases the difficulty of subsequent treatment and utilization, consumes a lot of manpower and material resources, and is difficult to implement on a large scale ^[10][11]. The shading technology method is mainly carried out by laying shading panels or shading cloth above the water surface to prevent the photosynthesis of algae to achieve the purpose of controlling algal blooms, but the technology is only applicable to the watershed area of small water bodies, since the watershed area of larger water bodies will cost a lot of human, material, and financial resources, so it has certain limitations. Air flotation is a method of solid–liquid separation by using algal flocs attached to tiny bubbles to float upward, which has a good removal effect on the algal solution with low concentration and low turbidity ^[11][12][12,13]. However, because the algae cells are negatively charged and have the same charge as the bubbles, it is not conducive for the algae cells to attach to the bubbles to produce inhibition, and the actual algae removal rate is difficult to reach 90%. Yap et al. ^[13][14] used a novel posiDAF dissolved air flotation process to modify the bubbles by the cationic polymer polyDMAEMA to make the bubbles positively charged and to remove algal cells up to 95% efficiently. As a natural and nontoxic substance, algae can be removed by flocculation and precipitation of clay, but this technique may cause water blooms to erupt again. Modified clay flocculation and sedimentation are ecologically fast and safe for algae removal, but there are disadvantages such as large clay dosage, high cost, and an unstable effect when using this technology alone. Ahmad et al. ^[14][15] explored the optimal conditions for the removal of algal cells by chitosan and found that the removal rate of algal cells reached 90% after 20 min of sedimentation at a chitosan concentration of 10 mg·L⁻¹, 20 min of stirring time, and 150 r·min⁻¹ of stirring speed. The ultrasonic method is a physical algae removal method developed in recent years, generally referring to the frequency of 20,000 Hz or more elastic mechanical waves, mainly using mechanical forces and cavitation effects generated by shock waves, high temperature, and pressure, jets, etc., to destroy the algal cell photosynthetic system and biological activity ^[15][16][16,17]. The ultrasonic method can further decompose algae cell secretions and algal toxins as well as other metabolic products; with good algae removal effects, simple installation and maintenance, fast removal speed, green environmental protection, and other advantages, it has a broad market prospect, but when ultrasonic algae control is used, the greater the ultrasonic intensity, the higher the energy consumption, the worse the economy, and the aquatic organisms are easily affected. Rajasekhar et al. ^[17][18] confirmed that the results showed that a 20 kHz ultrasound could kill algal cells rapidly and prolonged treatment can lead to exposure of microcystin, but ultrasound can remove microcystin simultaneously and selectively remove algae. Song et al. ^[18][19][19,20] demonstrated that microcystin can be rapidly degraded when the ultrasonic frequency is 640 kHz and that the microcystin (MC-LR) degradation pathway is the oxidation of the benzene ring by hydroxyl radicals and the diene bond on Adda peptide residues. In addition to this, the Mdha alanine peptide bond was also broken, probably by pyrolysis triggered by high temperatures generated at the cavitation site. Therefore, the main mechanism of ultrasonic degradation is hydroxyl radicals, and hydrolysis/cleavage processes are also present. Filtration methods generally include sand filtration and membrane filtration, which can remove algal cells intact without breaking the cells ^[20][21]. Sand filtration is usually used after coagulation and sedimentation or air flotation processes and is suitable for raw water with low concentrations of suspended matter and algae cells; raw water with high algae cell concentrations may clog or infiltrate the filter bed. The membrane filtration process is based on the selective permeation membrane as the separation medium, with the pressure difference or concentration difference driving the raw material side components selectively through the membrane, thus achieving the purpose of separation or purification. Among them, pressure-driven membrane filtration processes mainly include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). Among them, nanofiltration and reverse osmosis are high-pressure membrane filtration processes, which can effectively retain not only microorganisms and organic matter in water, but also ions and water-soluble salts in water to a large extent. This makes the water lack some beneficial trace elements and hardness and alkalinity, thus making it unsuitable for long-term drinking ^[21][22][22,23]. In addition, the high water purification efficiency of nanofiltration and reverse osmosis comes at the cost of correspondingly high operational energy consumption ^[23][24] and is not suitable for widespread application in water supply treatment. Microfiltration and ultrafiltration are both low-pressure membrane filtration processes with marketable material prices and operating costs, and have broad application prospects. However, microfiltration cannot completely retain pathogenic bacteria and viruses. Ultrafiltration technology, which is in the middle of microfiltration, nanofiltration, and reverse osmosis, makes good use of their respective advantages, and its membrane pore size is moderate, between 0.001 µm–0.02 µm, and its molecular weight is usually in the range of 1000–300,000 µm ^[24][25], so it can play a good role in the removal of some large molecules. MF and UF are effective in removing algal cells and microcystin ^[25][26]. Campinas et al. ^[26][27] achieved 93–98% removal of MCs by powdered activated carbon adsorption/ultrafiltration (PAC/UF) for the removal of microcystin. Ultraviolet (UV) irradiation can cause significant damage to the genes, cell membrane integrity, and photosynthetic capacity of algal cells, as well as inhibit the production and release of toxins and increase the sedimentation capacity of algal cells ^[27][28][28,29]. Alam et al. ^[29][30] irradiated Microcystis aeruginosa with UV light at a wavelength of 254 nm and found that algal cell growth was significantly inhibited for 7 d when the UV intensity was 37 mJ·cm⁻². When the irradiation intensity increased to 75 mJ·cm⁻², all algal cells died. Further probing revealed that UV significantly disrupted the photosynthetic system of the algal cells, causing severe damage to DNA strands and impeding protein transcription and synthesis. The adsorption method of algae removal is usually adsorption by activated carbon; powder activated carbon (PAC), granular activated carbon (GAC), and activated carbon fiber (ACF) are commonly used ^[30][31][31,32]. Activated carbon has a huge specific surface area and complex pore structure, which can easily adsorb organic substances with relative molecular masses between 500~3000 µm. Microcystins have a molecular weight close to 1000 µm and are easily adsorbed. Activated carbon is usually used as a coagulant in algae removal. Powdered activated carbon is added to strengthen the effect of coagulation and algae removal, while adsorbing microalgae, which is suitable for emergency treatment of high-algae water. Jiang et al. ^[32][33] developed an “activated carbon-membrane bioreactor” (PAC-MBR) to treat mildly eutrophic water bodies. After 4 months of continuous operation, it was found that algae, humic acid substances, and ammonia nitrogen (NH₄⁺-N) were effectively removed from the water. The mechanism of action, advantages, and disadvantages of physical algae removal techniques are shown in Table 2.